1Primate Research Institute, Kyoto University, 2Department of Zoology, Kyoto University, 3Department of Mechanical Engineering, Keio University
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Lorisids are characterized by its slow arboreal quadrupedal walk, and some adaptational inferences have been made on their limb morphology in relation to this unique locomotion. This study compared cross-sectional properties of humeral and femoral mid-shafts of lorisids with those of other primates, and reexamined previously suggested morphological characteristics of lorisids and its mechanical significances. The diaphyseal structure of long bones is suggested to be related to the stresses produced by locomotion and body weight support. Cross-sectional properties such as cortical areas, second moments of area, and section moduli were collected from the micro CT images (voxel size 0.02 - 0.05 mm). The sample included five lorisid species (Loris tardigradus, Arctocebus calabarensis, Nycticebus pygmaeus, N. coucang, and Perodicticus potto) and other small primates (galagids, lemurids, platyrrhines, and cercopithecids). We found that lorisids can be divided into two morpho types. The differences between two similarly-sized species, Nycticebus pygmaeus and Arctocebus calabarensis, indicated that the variation is not allometric. Perodicticus and Nycticebus show high cross-sectional property values, having a very high safety factor for their body mass and previously reported ground reaction forces. Loris and Arctocebus have longer bone lengths and smaller total subperiosteal areas relative to body mass than other primates, implying that their limb bones are weak against bending. However, because of the thick cortices, their cortical areas relative to body mass are equivalent to the other primates of similar body size; thus, their long bones have enough strength against axial loadings.
This study was funded by Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for JSPS Fellows (No. 15004748), for Scientific Research (B) (No. 16370104), for the 21st Century COE Program (A14 to Kyoto University), and for Advanced research Institutions in Japan (to Kyoto University), and by the Cooperation Research Program of Primate Research Institute, Kyoto University.